The present invention relates to the art of power driven threading machines and, more particularly to chucks for gripping a workpiece such as a rod or pipe and rotating the workpiece during the cutting, reaming and/or threading thereof.
Power driven thread cutting machines are well known and basically comprise a rotatable chuck assembly for supporting and rotating a workpiece, a tool carriage supporting tools for performing work in the rotating workpiece, and a drive unit for rotating the chuck assembly and thus the workpiece. The chuck assembly generally includes a tubular spindle having an axis of rotation and axially opposite ends, and a plurality of workpiece engaging jaw members pivotally supported at the opposite ends of the spindle for displacement radially inwardly and outwardly of the spindle axis between closed and open positions with respect to a workpiece introduced through the spindle. The jaw members on the spindle provide for a workpiece to be gripped at axially spaced apart locations along the length thereof and, in the closed positions of the jaws the workpiece is held in a centered position coaxial with the spindle. The spindle and jaws are rotatable as a unit to rotate the workpiece relative to the tool carriage which usually carries a cutting tool, a reaming tool and a thread cutting die head. When the jaws are in the open positions thereof, the workpiece is released and is adapted to rest on the inner periphery of the tubular spindle. The spindle diameter is such that the chuck assembly is adapted to grip and support workpieces having different diameters up to a given maximum diameter which is less than the inner diameter of the spindle. A drive unit is provided for the chuck assembly and, in connection with initiating operation of the threading machine, the jaws are generally pivoted relative to the spindle and into engagement with a workpiece positioned in the spindle, after which the workpiece and chuck assembly are driven together by the drive unit through the jaws. Upon completion of the cutting, reaming and/or threading operation, the direction of the drive is reversed, whereby the jaws pivot outwardly of the spindle axis to release the workpiece.
A variety of arrangements for pivotally supporting and driving the chuck jaws and chuck assembly have been provided heretofore. Such arrangements have included the individual pivotal mounting of the jaws by pins on a rotatable spindle component and the pivotal displacement of the jaws by a drive member coaxial with the spindle and having a camming interengagement with the jaws. An arrangement of this character is disclosed in U.S. Pat. No. 2,916,290 to Skillin. In other similar arrangements heretofore provided, as disclosed in U.S. Pat. No. 3,232,629 to Obear for example, the jaws are provided with gear teeth and are pivotally displaced by a drive member in the form of a ring gear. Both such arrangements require identical component parts at opposite ends of the spindle to drive the jaw members at the corresponding spindle end. Other arrangements heretofore proposed have provided for the jaw members at opposite ends of the spindle to be secured to opposite ends of a common drive rod pivotally supported by the spindle and pivoted by a structurally complex drive arrangement between the opposite ends of the rod. Such an arrangement is shown in U.S. Pat. No. 3,270,492 to Behnke and disadvantageously imposes bending forces on the rods in that the workpiece and chuck assembly are driven through the rods. Further, such an arrangement requires either an undesirably heavy supporting arrangement for the rods along the length thereof or undesirably large rods to accommodate the loads imposed thereon. In yet another arrangement heretofore proposed, as shown in U.S. Pat. No. 2,890,888 to Damijonaitis, the jaws are pivotally mounted on the spindle by means of corresponding pins, and the corresponding jaws at opposite ends of the spindle are pivoted relative to the spindle by means of common rods therebetween which are displaced circumferentially of the spindle by a drive member intermediate the opposite ends thereof. This arrangement also imposes undesirable bending loads on the rods in connection with the driving of the chuck mechanism with a workpiece therein.
All of the foregoing arrangements are undesirably heavy and expensive because of the structural complexity thereof. Additionally, many of the arrangements are subject to relaxing the grip of the jaws relative to the workpiece when the power is turned off, whereby the initially obtained positional centering of the workpiece relative to the spindle axis may be lost when the machine is restarted to complete work on the same workpiece. Most importantly in connection with the disadvantages of the foregoing drive arrangements is the fact that the gripping forces of the jaws against the workpiece at opposite ends of the spindle may not at all and in any event do not consistently either match one another or meet the requirements for supporting and rotating the workpiece with out slippage relative to the jaws. More particularly in this respect, if the workpiece gripping jaws at opposite ends of the spindle are actuated either by corresponding drive components or by common drive components, engagement of the jaws at one end of the spindle with the workpiece and with a given gripping force can be achieved prior to engagement of the jaws at the other end of the spindle with the workpiece to provide the same or a desired gripping force. In any event, such uneven gripping forces against the workpiece at opposite ends of the spindle promotes instability with respect to rotational support of the workpiece which can result in slippage of the workpiece relative to the chuck assembly during a threading operation and/or eccentric rotation of the workpiece and, consequently, the cutting of an unacceptable thread and possible damage to the thread cutting tools.
Yet another disadvantage with respect to chuck mechanisms heretofore provided resides in the fact that at least some manual assistance is required in connection with positioning the workpiece coaxially with respect to the spindle axis during displacement of the jaws from the open to the closed positions thereof about the workpiece. This is especially true in connection with workpieces which are of a small diameter relative to the inner diameter of the spindle. More particularly in this respect, if the workpiece is positioned at rest on the inner surface of the spindle, the jaws in moving from the open to the closed positions thereof either will not engage the workpiece, or engage the workpiece and jam in attempting to move the workpiece, and/or undesirably damage the workpiece or become damaged by engagement therewith. Either of the latter can result in misalignment between the workpiece and spindle axis when the workpiece is finally gripped and rotated by the jaws. Therefore, to avoid these potential problems, the workpiece is most often manually held in substantially centered relationship with the spindle during displacement of the jaws into driving engagement therewith.